The present invention relates to a method for treating industrial waste waters containing high levels of hexavalent chromium and other heavy metals, and more particularly to a method for efficient reduction of hexavalent chromium to trivalent chromium in a waste treatment process wherein acceptable sludge levels are produced as compared to processes known in the art.
Industrial waste treatment plants downstream from electroplating facilities are generally subjected to industrial wastes which contain relatively high levels of numerous toxic heavy metals in concentrations which often fail to meet National Pollutant Discharge Elimination Permit levels. Therefore, such waste water must be treated to reduce the levels of heavy metals to within discharge permit limits. The metals typically contained in these waste waters include chromium, cadmium, copper, lead, zinc, nickel and aluminum. Presently known processes for the treatment of such waste waters produce large quantities of metal-bearing sludges which are classified as hazardous wastes requiring special and costly handling and transport and disposal in hazardous waste landfills. While numerous processes exist for the precipitation of most heavy metals, hexavalent chromium (Cr.sup.+6) cannot conveniently be precipitated without first reducing the Cr.sup.+6 to trivalent chromium (Cr.sup.+3). The process for reducing Cr.sup.+6 to Cr.sup.+3 presently being used by a number of electroplating facilities utilizes sulfur dioxide in the reaction, EQU 2CrO.sub.4.sup.-2 +3SO.sub.2 +2H.sub.2 SO.sub.4 .fwdarw.2Cr.sup.+3 +5SO.sub.4.sup.-2 +2H.sub.2 O (1)
Other methods of acidic reduction of Cr.sup.+6 include use of sodium sulfite, sodium bisulfite and ferrous compounds. For example, the reaction utilizing sodium sulfite is, EQU 2CO.sub.4.sup.-2 +3Na.sub.2 SO.sub.3 +5H.sub.2 SO.sub.4 .fwdarw.2Cr.sup.-3 +8SO.sub.4.sup.-2 +5H.sub.2 O+6Na.sup.+ (2)
Additional methods of Cr.sup.+6 reduction include use of ferrous sulfate, sodium sulfide, a combination of both ferrous sulfate and sodium sulfide, and use of sodium borohydride. At acidic pH, the Cr.sup.+6 usually exists as HCrO.sub.4.sup.-, while at alkaline pH, hexavalent chromium exists as CrO.sub.4.sup.-2. Reduction with ferrous ion, such as with ferrous sulfate, at acidic pH proceeds as, EQU 3FeSO.sub.4 +HCrO.sub.4.sup.- +7H.sup.+ .fwdarw.3Fe.sup.+3 +Cr.sup.+3 +4H.sub.2 O+3SO.sub.4.sup.-2 (3)
The rate of Cr.sup.+6 reduction using an amount of ferrous ion substantially in excess of the stoichiometric amount proceeds faster than using a stoichiometric amount. The rate is based upon the reaction of HCrO.sub.4.sup.-. Sulfur compounds (S.sup.+4) can reduce Cr.sup.+6 at pH less than 3, the rate slowing logarithmically with increased pH. H.sub.2 S is the predominant specie at acidic conditions, while at neutral or alkaline pH conditions, the predominant species are HS.sup.- and CrO.sub.2.sup.-2. When ferrous ion is present as ferrous sulfate, Cr.sup.+6 is rapidly reduced at neutral and alkaline pH. The ferrous ion appears to catalyze the sulfide reaction. However, ferrous ion is not efficient by itself in reducing Cr.sup.+6 since only one electron is available per iron atom. A large quantity of iron hydroxide sludge is therefore produced. Ferrous ion and sulfide would appear to be the best combination for reducing and precipitating Cr.sup.+6 at neutral or near neutral conditions.
Metal precipitation by soluble sulfides require a sulfide source more soluble than the metal to be precipitated. Sodium sulfide dissociates readily into sodium and sulfide ions: EQU Na.sub.2 S+H.sub.2 O.fwdarw.2Na.sup.+ +S.sup.-2 +H.sub.2 O (4)
The free sulfide reacts with a heavy metal to form a precipitate. The metal sulfides that precipitate by this process can form extremely fine colloidal particles (pin floc). Under alkaline operating conditions, evolution of hydrogen sulfide gas is minimal.
It was previously proposed in U.S. Pat. No. 4,705,639 (Nov. 10, 1987) that a ferrous/sulfide process for reduction of Cr.sup.+6 and precipitation of Cr.sup.+3 and other heavy metals was possible in heavy metal contaminated waste water at pH of from about 8-10, and using about 90% stoichiometric sulfide and about 10-20% stoichiometric ferrous ion.
The insoluble sulfide (Sulfex.TM.) process uses freshly precipitated ferrous sulfide to precipitate heavy metals from a metal finishing waste stream. The freshly precipitated ferrous sulfide has substantially more reactive sites than pulverized iron sulfide and results in Cr.sup.+6 reduction and precipitation in one step (see U.S. Pat. Nos. 3,740,331 and 4,102,784).
It is therefore a principle object of the invention to provide an improved industrial waste treatment process.
It is another object of the invention to provide an improved waste water treatment process for removing heavy metals therefrom.
It is yet another object of the invention to provide a waste water treatment process for reducing contained hexavalent chromium to trivalent chromium and precipitation thereof from the waste water along with other heavy metals.
It is yet another object of the invention to provide a waste water treatment process for removing chromium and other heavy metals from the waste water with the accompanying generation of minimum amounts of sludge.
These and other objects of the invention will become apparent as the detailed description of representative embodiments proceeds.